In the fabrication of semiconductor devices, an important step is to provide electrical communication between an integrated circuit (IC) chip or die and the outside world. The communication may be established by a variety of packaging methods for the IC chip, for instance, wire bonding the IC chip with lead fingers on a lead frame, flip-chip bonding an IC die directly with lead fingers by solder bumps, etc. The wire bonding process for connecting an IC die to a lead frame has been widely used in the packaging of IC devices.
In the wire bonding process, a lead frame fabricated of a thin piece of metal such as copper or copper alloy is first provided. The lead frame may be patterned by a lithographic process to inform a multiplicity of lead fingers emanating from an outer frame toward a center of the lead frame. At the center of the lead frame, a die paddle which is a larger piece of the lead frame is also formed to provide a base for mounting an IC die thereto. A chemical etching process is normally utilized to form the multiplicity of lead fingers such that each of the finger has a base portion that is integral with the outer frame of the lead frame and a tip portion extending inwardly toward the center of the lead frame. The tip portions of the lead fingers are coined, or mechanically compressed in a final step to form a specific configuration of the tip portions. For instance, the tip portions normally bend into a flat portion forming a bonding site for wire bonding.
An automated wire bonding process is frequently used for providing electrical connection between an IC die and a lead frame. In the process, a high conductivity, thin metal wire such as one made of gold is used for the connection for achieving minimum resistance in the communication with the IC die. A capillary which is a wire bonding device with a wire threaded therein is used to plant a gold wire first on a bond pad on the surface of the IC die and then to the tip portion of a lead finger. A wedge bond with the leveled, top surface of the tip portion of the lead finger is thus formed. During the process of forming the wedge bond, the tip of the capillary automatically cuts off the wire supply such that it is ready for the next bond pad wiring connection The wedge bond formed on the lead fingers must exhibit a minimum bond strength requirement and furthermore, it must survive various adverse service environments for the IC package.
The IC die after wire bonded to a lead frame, is most likely encapsulated in a plastic molding compound. The encapsulation process can be performed in a plastic injection molding machine by positioning the IC die and the lead frame in a mold cavity. A plastic compound such as epoxy having a melt temperature of about 175.degree. C. is then injected into the mold cavity forming the encapsulation. The wedge bond formed between the bonding wire and the lead finger is therefore directly exposed to such high temperature or thermal shock. After the molding process is completed the molded package is removed from the mold cavity and allowed to cool down to room temperature. A high thermal stress is therefore exerted on the various bonding sites in the package, i.e., the various wedge bonds formed between the wires and the lead fingers, caused by the sudden temperature change and furthermore, by the shrinkage of the molding compound that surrounds the wedge bond. When the wedge bond is not formed with a high bond strength, the bond frequently fails in a phenomenum known as "second bond open" and thus causing the malfunction of the IC package.
In order to ensure the reliability of IC packages fabricated IC packages are tested in a standard reliability test known as the JEDEC thermal cycle test. In the JEDEC thermal cycle test, a temperature loading from 150.degree. C. to -65.degree. C. is performed on an IC package to qualify its resistance to thermal stress. Many IC packages do not pass the test due to a failure of the wedge bond formed between the bonding wire and the lead fingers, i.e., due to the second bond open defect.
FIG. 1 shows an enlarged, perspective view of a conventional lead frame 10, an IC die 12 and a heat sink 14. The lead frame 10 is constructed of an outer frame 16 which has a plurality of lead fingers 20 emanating from the outer frame 16 each equipped with a tip portion 22. It is seen that the lead fingers 20 and the die paddle support 24 for supporting a die paddle 26 are formed in a slope such that the die paddle 26 is at a plane below the plane of the outer frame 16. The tip portions 22 are formed in a stamping process into a coined area which is in the same plane as the die paddle 26. The die paddle 26 is further provided with apertures 32 for easier assembly of the IC die 12 to the heat sink 14 by adhesive means. The lead frame 10 is normally formed by a thin piece of metal such as copper or a copper alloy.
An enlarged, cross-sectional view of the lead frame 10, the IC die 12 and the heat sink 14 after a wire bonding and a plastic encapsulation process is shown in FIG. 1A. The tip 22 of the lead fingers 20 are connected to bond pads (not shown) on the IC die 12 by a highly conductive metal wire 34 after the IC die 12, the die paddle 26 and the heat sink 14 are first bonded together by adhesive layers 18. The adhesive layers 18 may be formed of an adhesive that is a metal particle filled thermoset material for providing improved heat transfer from the IC die 12. The wire bonded assembly is then positioned in a plastic molding cavity (not shown) and a plastic molding compound 36 is injected into the cavity to form the IC package 30.
An enlarged, detailed view of the wire bond formed between the IC die 12 and the lead finger 20 is shown in FIGS. 1B and 1C. As shown in FIG. 1B, a bond pad 38 is provided in a top surface 28 of the IC die 12 for providing electrical communication through a high conductivity wire 34 with the tip portion 22 of the finger lead 20. A more detailed view of the wedge bond 40 formed by the bonding wire 42, i.e., a gold wire is shown in FIG. 1C. A wire bonding ball 44 is also formed by the gold wire when the wire strikes the surface of the bonding pad 38. The wedge bond 40 formed on a leveled surface 46 on the tip portion 22 may not survive a thermal cycling test due to its high thermal stress.
It is therefore an object of the present invention to provide a lead frame for forming an improved wire bonded IC package that does not have the drawbacks or shortcomings of the conventional wire bonded IC packages.
It is another object of the present invention to provide a lead frame that is equipped with improved lead fingers capable of surviving a thermal cycling test after bonding to an IC die.
It is a further object of the present invention to provide a lead frame that has improved lead fingers with inclined tip portions for achieving improved wire bonding with an IC die.
It is another further object of the present invention to provide a lead frame that is equipped with lead fingers having tip portions formed of an inclined top surface with an angle smaller than 30.degree. when measured from a horizontal plane of the lead finger.
It is still another object of the present invention to provide a lead frame equipped with lead fingers that have an inclined tip portions with a top surface that is formed with an angle between about 5.degree. and about 20.degree. when measured from a horizontal plane of the lead finger.
It is yet another object of the present invention to provide an integrated circuit package that has an IC chip bonded to a lead frame by utilizing a lead frame equipped with lead fingers each having an inclined tip portion forming an angle of not more than 30.degree. when measured from a horizontal plane of the lead finger.
It is still another further object of the present invention to provide an integrated circuit package that has an IC chip bonded to a lead frame wherein the lead frame has lead finger with inclined tip portions such that after a gold wire is bonded to the inclined tip portion forming an interface, a thermal stress occurring at the interface is reduced by at least 20% when compared thermal stress obtained on lead fingers that have non-inclined tip portions.
It is yet another further object of the present invention to provide a method for establishing electrical communication between a lead frame and an IC die by providing a lead frame that has a plurality of lead fingers each equipped with a sloped tip portion having a top surface forming an angle of less than 30.degree. with a horizontal plane of the lead finger, and bonding the sloped tip portion to a bond pad on the IC die by a high conductivity wire.